1. Field of the Invention
The present invention relates to a white light emitting device, and more particularly, to a white light emitting device using up to two LEDs to achieve superior color reproducibility and high efficiency.
2. Description of the Related Art
In general, a white LED apparatus is preferred over a conventional small-sized lamp or a fluorescent lamp as a backlight of a Liquid Crystal Display (LCD). The white LED apparatus can be manufactured by forming a ceramic phosphor layer on a light exiting surface of a blue LED.
The most representative type of conventional white LED apparatus is realized by combining a GaN-based blue LED with YAG-based yellow phosphor. The blue light emitted from the blue LED excites the phosphor to emit yellow light. A mixture of blue light and yellow light is perceived as white light by the observer. Such a white LED apparatus has advantages like relatively high efficiency and low costs but also drawbacks like mediocre color reproducibility.
FIG. 1 shows a light emission spectrum of a conventional white light emitting device manufactured by combining a GaN-based blue LED with YAG-based yellow phosphor. As shown in FIG. 1, the light intensity is high in the blue and yellow regions but low in the green and red regions. Therefore, such a white light emitting device does not well express the green and red regions, thus having a mediocre color rendering index and color reproducibility.
Alternatively, a white light LED apparatus may be realized by combining blue, green and red LEDs. FIG. 2 shows light emission spectrums of such a type of conventional white LED apparatuses composed of blue, green and red LEDs. In particular, FIG. 2 shows light emission spectrums of white LED apparatuses with different correlated color temperatures. Such white LED apparatuses have relatively good color reproducibility but with high power consumption and low efficiency. Moreover, these white LED apparatuses are not cost-effective with complex circuit configurations.
As a further alternative, LEDs can be used to realize a white light emitting device by combining an ultraviolet LED (UV LED) with green/blue/red phosphor or a mixture of blue, green and red phosphors. Such a white light emitting device has superior color reproducibility with low costs, but is not yet commonly used and has low efficiency. Therefore, there exists a need for developing a white light emitting device which satisfies a need for high efficiency and high color reproducibility.